Conspecific brood parasitism and egg quality in blue tits Cyanistes caeruleus

Laying eggs in nests of unrelated conspecific pairs to parasitize their parental care is a common phenomenon in birds. In blue tits Cyanistes caeruleus such conspecific brood parasitism (CBP) has never been reported in the literature. However, in a situation where breeding density was extremely high, we found six nests to be parasitized with eggs of conspecific females. Natural selection may favour elevated competitiveness of parasite young, since the negative consequences of increased sibling competition are incurred on the unrelated host parents and siblings, and therefore do not act as inclusive fitness costs for the parasites. Parasitizing females could achieve such a competitive advantage for their offspring by laying larger eggs or eggs with higher concentrations of testosterone in the yolk. We analyzed these parameters of the six parasitized nests, but did not find that parasite eggs differ systematically in these aspects from host eggs, nor that parasite eggs showed resemblance to host eggs. We suggest that a shortage of available nest sites caused some females to use CBP as a best‐of‐a bad job strategy, but that either the occurrence of CBP is too rare to lead to strong selection for egg adjustments or that parasitizing females are unable to do so.